Japanese Patent Publication No. 18069/1972 and Japanese Patent Laid Open Publication No. 82077/1976 disclose techniques very pertinent to the present invention. However, they do not teach the laminated nonwoven fabric of the present invention, particularly, the mode of bond of the laminate plys.
That is, Japanese Patent Publication No. 18069/1972 discloses the production of a nonwoven fabric of entangled fibers obtained by injecting an incompressible fluid in the form of a columnar flow having a momentum flux of not less than 6 kg m/sec.sup.2 cm.sup.2. This publication also discloses that, after the treatment of the columnar flow, the resulting nonwoven fabric is subjected to a dry or wet heat treatment to develop crimp and self-extensibility of web-forming fibers and to make the entire web more bulky. Further, Examples of this publication disclose not only a single-ply web having a fabric weight of about 10 to 400 g/m.sup.2 but also a two- or three-ply web. Particularly, Example 30 thereof discloses a laminated nonwoven fabric having a sandwich structure of three-ply web having the middle ply of which is made of a fiber having a lower softening and melting points (nylon) and each of the outer two plys of which is made of a fiber having a higher softening and melting points (polyester). However, this publication does not teach the heat treatment of the present invention wherein only a web-forming fiber composing one laminate ply is softened or molten and laminate plys are bound to each other by partial softening or melting of a fiber having a lower softening and melting points without softening or melting of a fiber having a higher softening and melting points as in the present invention. In fact, in practice, there is a possibility of cleavage of plys of the nonwoven fabric of this publication and fluff of fibers on the surface thereof is insufficiently improved.
Japanese Patent Laid Open Publication No. 82077/1976 discloses to subject a laminated web to a fluid entangling treatment and partial softening or melting of web-forming fibers. However, this publication discloses only a laminated web made of the same kinds of fibers. That is, this publication merely discloses the treatment of a laminated web made of the same kinds of fibers by an entangling treatment with fluid and then heating. Accordingly, fibers which forms adjacent both plys of web are uniformly softened or melted, and hence, the resulting nonwoven fabric is liable to become flattened and hard.
In order to obtain a laminated nonwoven fabric having improved characteristics which hardly shows cleavage of laminate plys and hardness of tissue, the present inventors have intensively studied. As the result, it has been found that a desired laminated nonwoven fabric having excellent characteristics is obtained by employing different kinds of fibers having different softening and melting points of which hardly undergoes the effect of heat during a heat treatment of laminate plys of web.